1. Field of the Invention
This invention relates generally to video signal reproducing devices, and more particularly is directed to improvements in such devices for modulating the scanning velocity of the electron beam or beams for reproducing a picture image having significantly improved sharpness.
2. Description of the Prior Art
When the phosphor screen of a video signal reproducing device, as in the cathode ray tube of a television receiver, is scanned by an electron beam or beams so as to form a picture image on the phosphor screen, the electron beam forms on the phosphor screen a beam spot whose size is larger at high brightness levels than the spot formed by the electron beam corresponding to the low brightness level portions of the image. Further, when a beam scanning the screen moves across the demarcation or edge between image areas of low and high brightness, for example, black and white areas, respectively, the beam intensity cannot switch instantly from the low level characteristic of the black area to the high level characteristic of the white area, with the result that the sharpness of the reproduced image is degraded at portions of the image where sudden changes in brightness occur in response to transient changes in the luminance or brightness of the video signal being reproduced.
As means of compensating for such degradation of the apparent response of the picture image, an aperture compensation technique has been proposed and is described, for example, in R. C. Dennison, "Aperture Compensation for Television Camera", RCA Review, 14,569 (1953). In accordance with such aperture compensation technique, the intensity of the electron beam is first decreased and then increased at those portions of the picture image at which the brightness changes from a low level to a high level. Although this procedure actually increases the time required for transition of the beam intensity between its low and high levels, there is created a visual edge effect which, to some extend and in the case of relatively small screens, registers psychologically as improved edge sharpness. However, such compensation is insufficient for achieving really sharp definitions between light and dark areas of a reproduced image, particularly in the case of relatively large screen areas.
In order to avoid the above disadvantage of the aperture compensation technique, it has been proposed to detect transient changes in the brightness level of the video signal being reproduced and to change the electron beam scanning velocity from its normal velocity in reponse to the thus detected transient changes. Devices employing the foregoing so-called velocity modulation have been described, for example, in U.S. Pat. No. 2,227,630, No. 2,678,964 and No. 3,752,916.
However, difficulties are encountered in respect to the means employed for modulating the electron beam scanning velocity from its normal velocity. For example, in each of U.S. Pat. No. 2,227,630 and No. 2,678,964 it is proposed to supply the control signal for effecting the beam scanning velocity modulation to an additional deflection coil provided on the neck portion of the cathode ray tube so as to produce a magnetic field for suitably influencing the scanning velocity. However, by reason of the inductance and capacitance inherent in such additional deflection coil, the beam scanning velocity cannot be modulated at the required frequency. Although the foregoing problem is overcome by employing electrostatic deflection of the beam to achieve the scanning velocity modulation, for example, as also disclosed in U.S. Pat. No. 2,678,964 and as disclosed in U.S. Pat. No. 3,752,916, the arrangement proposed in these patents are disadvantageously limited to effective use only in monochrome or black-and-white television receivers and, in any case, require elongation of the cathode ray tube neck and consequently undesirably increase the depth of the television receiver cabinet. In each of U.S. Pat. No. 2,678,964 and No. 3,752,916, the control signal for effecting velocity modulation is applied across additional deflection plates disposed in the cathode ray tube neck, preferably in advance of the electromagnetic or electrostatic field by which the electron beam is focused at the phosphor screen. It will be apparent that the electrostatic deflection of the beam in response to such control signal for achieving the desired velocity modulation causes the electron beam to deviate from a path through the center of the focusing field at which minimum aberrations are imparted to the focused beam. Although such aberrations may be tolerable in the case of a monochrome television receiver, the problem is accentuated in the case of a color television receiver in which three electron beams are usually employed. In the case of color television receiver employing three electron beams originating at spaced apart points and being directed to impinge on respective color phosphors of the screen after passing through either a common focusing field or individual focusing fields, the electrostatic deflection of the beams in advance of such focusing field or fields gives rise to undersirable aberrations in the focused beams. Further, since the beams are subjected to electrostatic deflection to achieve velocity modulation at locations where the beams are relatively widely spaced apart, individual sets of deflection plates have to be provided for the three beams with the result that the structure is undesirably complicated.
In U.S. Pat. No. 3,830,958, issued Aug. 20, 1974, and having a common assignee herewith, arrangements are disclosed for effecting beam scanning velocity modulation in color video signal reproducing devices, particularly of the type disclosed in U.S. Pat. No. Re. 27,751, and which are known as color cathode ray tubes of the Trinitron (Trademark) type. In such devices of the Trinitron (trademark) type, the three electron beams for energizing respective phosphors arranged in groups on the screen are made to converge from laterally spaced apart points of origin so as to intersect each other at a location which is substantially at the center of the main focusing lens and to exit from the latter along divergent paths with which convergence deflecting means are associated for reconverging the electron beams to impinge on the respective phosphors in one of the groups thereof, and a deflection yoke is provided on the color cathode ray tube between the convergence deflecting means and the screen for causing the beams to scan the screen in line-scanning and vertical direction. In U.S. Pat No. 3,830,958, it is proposed that velocity modulation be effected in color video signal reproducing devices of the foregoing type by means of a single pair of deflection plates spaced apart in the line-scanning direction and being disposed immediately adjacent the main focusing lens between the latter and the convergence deflecting means so that the three beams pass between the deflecting plates at portions of their respective divergent paths that are relatively close to each other. Therefore, when a control signal is applied across such pair of deflection plates in response to transient changes in the brightness of the video signal being reproduced, the resulting electrostatic field deflects the three beams more or less equally for achieving the desired velocity modulation. However, it will be apparent that the effect of the electrostatic field on the three beams is not precisely equal by reason of the spacing between their respective paths through such field. Further, the deflecting plates added to the electron gun for achieving the described velocity modulation necessarily increases the length thereof so that the depth of the cabinet cannot be minimized.
In previously existing circuits for providing the control signal by which beam scanning velocity modulation is effected in response to transient changes in the luminance or brightness component of the video signal being reproduced, a plurality of delay lines are employed. For example, in U.S. Pat. No. 3,830,958, a luminance signal separated from the color video signal being reproduced is applied to the input end of a first delay line and also to a subtractor while the output from the first delay line, that is, the once delayed luminance signal, is applied to the input of a second delay line to provide, at the output of the latter, a twice delayed luminance signal applied to the subtractor so that the output of the latter is the difference between the separated luminance signal and the twice delayed luminance signal and consitutes the control signal for the velocity modulation. The once delayed luminance signal from the first delay line is also applied to the matrixing circuit which receives color difference signals and which is operative to provide respective color signals by which the beam intensities are controlled. However, since delay lines are relatively costly components, it is desirable to reduce the number of such components required for producing the control signal by which velocity modulation is effected.